The October issue of the MRS Bulletin focused on two areas of research in biomolecular materials; biomineralization and protein fibers. In addition, the development of a new carbohydrate-based material with a variety of novel properties was discussed.

In this issue, we turn to two other types of materials based on biological systems: (1) polymers and other materials produced through enzyme-catalyzed biosynthetic reactions and (2) protein and lipid complexes based on biological membrane structures. In each case, as in the October issue, a discussion of how Nature produces and uses these materials is followed by reports describing manipulations of these systems to enhance their properties for nonbiological applications.

In the October issue, Kaplan and Cappello discussed in detail the synthesis of proteins. They described how living organisms expend a very large fraction of their energy producing these polymers to grow and repair damage to their bodies. (The other major fraction of their energy is dedicated to maintenance of brain function). Synthesis of carbohydrates, lipids, and other materials, however, is achieved through a somewhat more complex process. The organism outlines a pathway that consists of a sequence of reactions; each reaction converts a substrate-the product of the preceding reaction-to a product which is the substrate for the next reaction. The net result of the action of the pathway is the conversion of a nutrient to a useful material, often of far different chemical structure.

A wide variety of microorganisms accumulate an optically active polymer of D(−)-3-hydroxybutyric acid, P(3HB), as an intracellular storage material of carbon and energy. The P(3HB) was first isolated from Bacillus megaterium in 1925 by Lemoigne. Many prokaryotic organisms, such as bacteria and cyanobacteria, have been found to accumulate P(3HB) up to 80% of their cellular dry weight when growth is limited by the depletion of an essential nutrient such as nitrogen, oxygen, phosphorus, or magnesium. Recently, many bacteria have been found to accumulate copolymers of D(−)3-hydroxy-alkanoic acids with a chain length ranging from three to 14 carbon atoms. In addition, 4-hydroxybutyric acid was found as a constituent of bacterial polyhy-droxyalkanoates (PHA).

Bacterial PHA polymers have attracted much attention as environmentally degradable thermoplastics for a wide range of agricultural, marine, and medical applications. PHA is degraded in soil, sludge, or sea water. Some microorganisms such as bacteria and fungi secrete extracellular PHA depolymerases to degrade environmental PHA and utilize the decomposed compounds as nutrients. In this article, I report the metabolism, production, and properties of bacterial PHA.

Plants and animals have been exploited as sources of materials for centuries but, as our ability to analyze and fractionate them has progressed, the extraordinary range of properties available from materials produced by living systems has continued to grow. Doi, in another article in this issue of the MRS Bulletin, presents a discussion of a group of naturally occurring polyesters related to poly(beta-hydroxybutyrate). These polyesters are formed in vivo by several microorganisms as part of an energy storage scheme. Research on these systems has allowed growth conditions to be found that can lead, in a controlled fashion, to a number of copolymers. Useful materials based on these bacterial polyesters appear to be at hand.

The in-vivo formation of polyesters in microorganisms also illustrates several of the important reasons for examining biocatalytic polymer synthesis. First, unlike most industrial syntheses of polyesters, the poly(beta-hydroxybutyrate) biosynthesis occurs at a near-ambient temperature using a carbohydrate feedstock. Second, and perhaps most importantly, the stored polyesters are readily biodegraded by the bacteria that manufacture them, so materials based on these polyesters should also be biodegradable. Third, although there are side chains along the polymer backbone, they are introduced in a highly stereo-specific fashion during in-vivo synthesis, leading to an entirely stereoregular polyester. However, along with these advantages, there are also significant limitations to bacterial polyester synthesis. First, there are some substrates that are not incorporated into the polyester by the bacteria. Second, normal metabolism leads to the polyester, always incorporating a fraction of hydroxybutyrate monomers. Third, the backbone is always comprised of four-atom, A-B type 3-hydroxy acid repeat units with variations appearing in the side chain at carbon-3.

Phenomenal rate accelerations, exacting selectivities, and mild reaction conditions characterize biocatalysis and have generated considerable interest in nonbiological applications of enzymes. As a result of greater availability and purity, these molecules are now being used increasingly in organic synthesis to prepare complex natural products and novel materials. They are also being exploited industrially in the production of food additives, pharmaceuticals, and fine chemicals.

Unfortunately, for many commercial applications, natural enzymes may be unsuitable. They may be unstable or difficult to isolate; they may function poorly at the temperatures, pH's, and substrate concentrations needed for reaction; or they may lack appropriate specificity. For many chemical reactions, a natural enzyme may not even exist. For example, natural biocatalysts for the synthetically valuable Diels-Alder cycloaddition have yet to be discovered.

All living cells, and many of the structures within these cells (mitochondria, nuclei, chloroplasts) are surrounded by biological membranes which serve to separate the cell contents from the surrounding environment. The biological membrane is an extraordinary material. It controls the highly selective transport of molecules into and out of the cell. It senses the environment outside the cell and transmits information about it to the intracellular machinery. It reports information about the cell to the outside world—its identity and its state of function. It transports electrons, converts sunlight to chemical and electrical energy, pumps small molecules against a concentration gradient, and uses that gradient as a source of energy. The membrane is a generally robust structure, and one that can be modified in a controlled manner, making it adaptable for use in nonbiological applications. It has served as a model for sensors and detectors, for surface modification agents, for drug delivery systems, and for information storage and delivery, as well as other optoelectronic functions.

In 1971, the first halobacterial retinal protein was discovered and named bacteriorhodopsin. Bacteriorhodopsin (BR) is a protein of 248 amino acids that is integrated into the bacterial membrane. It is a light driven pump, moving protons from the inside to the outside of the cell. The resulting gradient drives the production of ATP (adenosine triphosphate) for the organism. In the 1970s, rapidly growing research activities focused on the biochemical and biophysical properties of BR and resulted in a linearly increasing number of publications per year. The constant level of about 200 publications/year in the last decade signifies the steadily growing knowledge about the structure and bioenergetic function of BR and related molecules. Nowadays, BR is one of the best-investigated membrane proteins (e.g., References 3–10).

Only five years after the unusual chemical and photophysical properties of BR were reported, several articles appeared suggesting technical applications of BR for sunlight energy conversion. Although no commercial applications of BR have resulted from these studies, new ideas for application have arisen, covering about 10% of the total publications on BR during the last two years. Is this an indicator of real potential for BR as a biological material in technology?

Biological membranes provide numerous functions for the survival of cells, ranging from molecular recognition to signal transduction and energy conversion. The spacial organization of proteins, enzymes, glycoproteins, and glycolipids in the membrane is provided by the lipid bilayer matrix. The lipids of the membrane are small molecules which have the common characteristic of having both a hydrophilic and a hydrophobic moiety. In aqueous media, they are entropically driven to self-organize in bimolecular sheets (see the article by Alper in this issue). These sheets, in addition to providing a fluid matrix for a wide variety of biomolecules, also serve to impede the flow of molecules across the membrane.

The progressive miniaturization of electronics is familiar to everyone; computing power that used to occupy a room can now be put onto a single integrated circuit. Miniaturization is admirable when size is not relevent to the electronic function, but there are plenty of devices in which a large physical size is essential. Many of these concern information technology, examples being the computer display, printer, and fax machine. Communication with the electronic world is primarily visual and operates at a large format typified by a document. In this rapidly advancing field, we are eagerly awaiting desktop interactive display surfaces that can input as well as display information, document readers, truly portable computers, electronic vision and—the oldest dream of all—the television that hangs on the wall. Realization of these hopes requires a set of technologies that permit bigger rather than smaller electronic systems, and amorphous silicon is helping to make this happen.